The present invention relates to electric marine propulsion systems. Specifically, an AC electric motor drive is provided for a fixed pitch propeller wherein full power is available at variable shaft RPM and torque.
Marine propulsion systems are of two basic types with regard to the structural design of the propeller. The first basic type is a propeller of "fixed pitch", usually solid cast, wherein the propeller RPM is controlled to comply with the demands of the system. Electric motor drives for fixed pitch propellers are described in various references, including U.S. Pat. No. 4,114,555, and in a paper entitled "SCR Controlled Electric Propulsion System" by Harry W. O'Brien, Jr., delivered at the spring meeting of the Society of Naval Architects and Marine Engineers in New Orleans, La. on Apr. 1, 1977. These referenced systems generate alternating current, and subsequently convert the AC current to DC for driving a DC motor. The prime mover driving the alternator is usually a diesel engine operated at a governed speed, which can produce maximum horsepower upon demand. As such, the prime mover is operated at its continuous rated speed to produce, upon demand, its rated horsepower output. These systems typically control motor RPM by varying the applied DC voltage through the use of an SCR controller. As such, the variable DC voltage supplied to the motor determines the propeller shaft RPM and shaft horsepower, as required. The DC motor is usually coupled to the propeller shaft through a reduction gear box.
The foregoing referenced electric propulsion system provides for control of propeller shaft RPM over a first and second operating speed range, thus producing constant torque up to 100% of base power and shaft RPM, and a variable reducing torque, at 100% of base power, while operating at a shaft RPM above base RPM. The horsepower output of the propulsion system also varies proportionately with changes in shaft RPM up to 100% of base power, thus providing a variable horsepower, constant torque system, up to 100% of base power and shaft RPM. A constant horsepower reducing torque system is provided once full horsepower output at 100% of base horsepower is attained at the top of the first range of operational RPM, wherein horsepower remains constant during operation of the vessel, from the bottom to the top of the second range of operational RPM. The foregoing electric propulsion system is extremely expensive and is inherently burdened with a large mass to power ratio.
Al alternative to the fixed pitch propeller, DC electric drive is the non-electric controllable pitch propeller drive, usually of precision machined fabricated design, which provides the availability of full engine horsepower at a constant propeller shaft RPM. During operation of a controllable pitch propulsion system, the engine can maintain its full horsepower and RPM while complying with the system's various load demands by increasing or decreasing the pitch of the propeller. The advantage of the controllable pitch propeller system is a fully mechanical power train, with the ability to maintain 100% full horsepower while operating under the various load conditions experienced during operation of the vessel.
The controllable pitch propulsion system, however, incurs proportionately excessive frictional drag losses when operating under low pitch, high slip conditions, which diminish the overall propulsive efficiency of the propeller system. As the pitch of the propeller is reduced while shaft RPM and power remain constant, the horsepower which is sacrificed to overcome frictional drag losses becomes very large, when compared to the greatly reduced frictional drag loss of a fixed pitch propeller capable of operating at much lower RPM, while still maintaining full engine RPM and horsepower output. The present invention combines the advantages realized in controllable pitch propulsion systems with those realized in DC or AC-DC electric propulsion systems, while eliminating their respective disadvantages. The system of the invention can provide a constant horsepower output to a fixed pitch propeller over a wide range of shaft RPM to affect a variable torque system which substantially reduces propeller frictional drag losses while operating at low shaft RPM, full power and high propeller slip. The electric propulsion system of this invention provides substantially 100% horsepower over the full useful RPM range of the propeller, while maintaining optimum utilization of the available horsepower.